The present invention relates to a flat-panel display used as an image monitor for a computer and a television receiver and particularly to a liquid crystal display driven by a signal voltage whose polarity is periodically reversed.
In recent years, liquid crystal displays have been widely used in views of merits of thickness, light weight, and low power consumption. The liquid crystal display has a structure in which a liquid crystal layer is held between an array substrate and a counter substrate. Each of the array substrate and the counter substrate, for example, has a light transmitting and insulating property, and the liquid crystal layer is made of liquid crystal composition filled into a gap between the array substrate and the counter substrate. The array substrate comprises a matrix array of pixel electrodes, a plurality of scanning lines formed along columns of the pixel electrodes, a plurality of signal lines formed along rows of the pixel electrode, and a first alignment film covering the entire matrix array of pixel electrodes. The scanning lines serve to select the corresponding rows of the pixel electrodes, and the signal lines serve to apply pixel electrode signal voltages to the pixel electrodes of the selected row. The counter substrate has a counter electrode facing the matrix array of pixel electrodes, and a second alignment film covering the entire counter electrode. The first and second alignment films are provided for causing liquid crystal molecules of the liquid crystal layer to be set in a twisted nematic (TN) alignment when no potential difference exists between the pixel electrode and the counter electrode. When light is incident to the liquid crystal layer from one substrate side through a polarizing plate, light rotates along the twist of the liquid crystal molecules aligned in the thickness direction of the liquid crystal layer, so as to be guided to the other substrate, and selectively transmitted through a polarizing plate. If a potential difference is provided between the pixel electrode and the counter electrode, the molecules are tilted up by an angle, which is proportional to the potential difference, from the plane parallel to the substrate surface where an image is displayed. As a result, light transmittance is changed.
In an active matrix liquid crystal display, a plurality of thin film transistors (TFT) are respectively formed near intersections of the scanning lines (or gate lines) and the signal line (or data lines), and each used as a switching element for selectively driving the corresponding pixel electrode. Each TFT has a gate connected to one scanning line, and a source-drain path connected between one signal line and one pixel electrode. The TFT is turned on in response to a rise of a scanning pulse from the scanning line, and supplies the pixel signal voltage to the pixel electrode from the signal line. The pixel electrode and the counter electrode are associated with the liquid crystal layer to constitute a liquid crystal capacitance to be charged according to the potential difference between these electrodes. This potential difference is maintained by the liquid crystal capacitance even after the TFT is turned off in response to a fall of the scanning pulse.
In a case where the electric field is kept in the same direction, materials other than the liquid crystal tend to gather one electrode side, thereby causing the life of the liquid crystal layer to be shortened. Conventionally, a technique of reversing the polarity of the pixel signal voltage with respect to the potential of the counter electrode every one frame period, for example, is known as a solution of the problem. If the polarity of the pixel signal voltage is reversed in the same manner for all the pixel electrodes during the frame period, this causes generation of flickers which deteriorate the image quality. To reduce the flickers, there is used a drive method of driving adjacent columns of the pixel electrodes by the pixel signal voltages of the different polarities. For example, for a certain frame period, pixel signal voltages of the negative polarity are applied to the pixel electrodes connected to the even-numbered signal lines, and signal voltages of the positive polarity are applied to the pixel electrodes connected to the odd-numbered signal lines. For a next frame period, pixel signal voltages of the negative polarity are applied to the pixel electrodes connected to the odd-numbered signal lines, and pixel signal voltages of the positive polarity are applied to the pixel electrodes connected to the even-numbered signal lines.
There is also known a drive method of further driving adjacent rows of the pixel electrodes by pixel signal voltages of the different polarities. For each frame period, pixel signal voltages of the positive polarity are applied to the odd rows of the pixel electrodes connected to the odd-numbered signal lines, and the even rows of the pixel electrodes connected to the even-numbered signal lines. Moreover, pixel signal voltages of the negative polarity are applied to the odd rows of the pixel electrodes connected to the even-numbered signal lines, and the even rows of the pixel electrodes connected to the odd-numbered signal lines.
With this drive method, the polarity of the pixel signal voltage is reversed for each of the pixel electrodes arranged two-dimensionally on the liquid crystal display screen. As a result, the flickers can be prevented from being visually recognized easily.
However, a voltage of about .+-.5 V is normally needed to control the liquid crystal. Due to this, it is necessary for a signal line driver to have a driving ability which can obtain a sufficient voltage accuracy in a large output dynamic range of 10 V. This causes an increase in power consumed by the liquid crystal display.